Basic ISDN service allows a telecommunications customer to have access simultaneously to two 64 kbps (kilobits per second) communication paths ("B" channels) and one 16 kbps path ("D" channel) on a switched telecommunications network. The D channel is a packet traffic channel which includes signaling for the two B channels, whereas the B channels can be either circuit or packet traffic channels. Accordingly, the request for service on a B channel would be made in packet form on the D channel, along with such signaling information as the destination address and the type of traffic on the B channel, as well as general packet data transmission.
Basic ISDN can be served by a telecommunications supplier bringing three logically separate circuit paths (two B's and one D) to the customer's premises, or multiplexing the circuits onto a carrier system. In the latter case, the customer's three circuits would be brought from the customer premises to a remote terminal, where the three circuits could be combined with other customers' circuits (ISDN or other services) and multiplexed together to be transported via carrier systems to the supplier's central station. For example, "T-carrier" could be utilized to transport 24 circuits (for example, 8 ISDN customers) on two pairs of wire from the remote terminal to the central station, thereby reducing the cost of transport compared with 24 pairs of wire from various customer premises to the central station.
The carrier system approach can either be dedicated or concentrated. For example, a T-carrier system could either be established to service 24 customer channels (dedicated), or could serve 50 customer channels on a first-come first-served basis (concentrated). The dedicated system may be more costly, but no traffic requests would be denied. The concentrated system would require less transmission facilities to serve an equivalent number of customer lines, but service might be denied in some cases. For instance, if 25 of the 50 customer channels had service requests in the concentrated example above, one of the requests would be denied, as the T-carrier system could only accommodate 24 channels.
Concentration schemes have been proposed for ISDN wherein a fixed number of channels would be set aside for D channel use, and the remaining channels would be set aside for B channel use. This allocation process has been suggested for two basic reasons. First, since the D channel is 16 kbps, and a carrier channel can handle 64 kbps, four D channels can be multiplexed onto one carrier channel. Also, since requests for B channel connections and disconnections are transmitted over the D channel, if B channel traffic used up the carrier system's capacity, D channel requests would be blocked, including requests for
The problem with such an allocation process is that it may be inefficient and prone to blockage. If the B channel allocation is used up, and the D channel denied, even though the T-carrier system would not be at full capacity. Conversely, the D channels could be fully utilized, and requests for additional D channel service would be denied even though spare B channels are available.
Adaptive concentration schemes (usually known as "Movable Boundary" techniques) have been previously proposed for handling combinations of circuit traffic and packet traffic over the same network (e.g., M. J. Fischer and T. C. Harris, "A Model for Evaluating the Performance of an Integrated Circuit- and Packet-Switched Multiplex Structure", IEEE Transactions on Communications, Vol. COM-24, No. 2, February 1976). The application of these techniques to ISDN networks has not been previously envisioned. There are two basic reasons why this has not been contemplated before. The first is that, historically, circuit and packet transmissions were independent of one another. Therefore, the object was to handle as many circuits in the network as possible with no interaction between the two types. With ISDN, as previously discussed, requests for circuit transmissions (B channels) are made over the packet circuits (D channels). Standard movable boundary techniques would not address this situation, because standard techniques do not allow for the interdependencies of circuits. Requests for service would be made over the same circuit as the service. Where requests for service arrive over a different circuit, the adaptive concentration switch must translate messages on one circuit in order to determine the demands for service on other circuits.
Second, the standard movable boundary situation involves a single switching point with a limited amount of outgoing circuit capacity, and therefore would adaptively allocate the circuit and packet demands at that location. ISDN is a much more complex situation. The limited circuit capacity is between two offices (remote terminal and central station). Further, the queuing of packet channel requests for B channel service would be received and interpreted at the central station, and the remote terminal is required to physically switch the customer lines to available channels on the communications link between the remote terminal and the central station. A standard adaptive concentration switch would simply link incoming channels with outgoing channels, while queuing data from packet channels. In other words, a standard adaptive concentration switch would be but one node in a network.